1. The Field of the Invention
This invention relates to metal detectors of the type used to detect metal fragments in foodstuffs and pharmaceuticals, and is directed more particularly to an improved search coil assembly for such metal detectors.
2. Description of the Prior Art
In many industires, such as food processing, non-metallic materials, such as foodstuffs, are conveyed automatically to various processing machines and stations. A problem arises when tramp metal is introduced into the material stream. Such tramp metal can constitute a hazard if sold to the public in foodstuffs and even in the processing area can constitute a hazard for some types of processing machinery.
It is known in the art that the non-metallic material can be passed through an electromagnetic field of a search coil and that if tramp metal is present in the material stream such metal will disturb the electromagnetic field of the search coil and induce a detectable error signal in the search coil. Responsive to the error signal, the material stream can be stopped or diverted to a waste container, accompanied, if desired, by visual and audible alarm means.
The sensitivity of such search coil metal detector systems is related to the size of an aperture through which the non-metallic stream is directed and around which is disposed the search coil. The smaller the aperture, or passage hole, for the product under test, the smaller is the size of metallic particles that can be detected. It has, therefore, been common practice to select an aperture size which allows the product, and its conveying belt, to pass through the aperture with minimum clearance.
For example, if it were required to inspect a packaged food product whose outside dimensions were 4".times.5".times.10", with the 10" dimension occurring in the direction of travel, the aperture size chosen probably would be on the order of 5".times.7". Since package or product size is a result of particular marketing and manufacturing requirements peculiar to each product at a given time, the resultant effect has been to require a very large number of different sizes and shapes of metal detectors to be maintained in inventory, or to require an extensive time delay in the making of a custom metal detection machine for a new sized product.
The size and shape requirements have resulted in a difficult manufacturer--end user relationship. The manufacturer must maintain a large number of different sizes, many of which may be inventoried for an uneconomical period of time. The alternative, as mentioned above, is to build to order. Because of the laborious manner in which the search coil assemblies are made, producing a particular size and/or shape to order is quite expensive and time-consuming.
Because of the expense involved, it is customary that such detectors are used only at processing plants or distribution points. However, at times metal is introduced into the product after the product leaves the processing plant and before sale to a consumer. Such is likely to be the case in occasions of "tampering".
The present state of the art in the manufacture of metal detector search coil assemblies requires the production of a metal housing to serve as a metallic shield around the electrical coils. When the correct size frame is made, the internal walls thereof are coated with graphite and over the graphite, a coating of plastic. On the external walls, the conductive strands are wound on the non-metallic frame which is then attached to a rigid metal housing which provides an enclosed void around the exterior of the frame and strands. The space between the strands and the housing is then filled with non-metallic filler, such as an epoxy. The filler acts as a moisture sealant, as well as a retainer for reducing the relative motion between the metal housing and the strands. Any relative motion between the strands and the surrounding metal housing results in a spurious signal.
The above-described method requires, in ordinary practice, about ten to thirty weeks to produce a detector apparatus ready for commercial use, an obviously lengthy and very expensive method of production.
Accordingly, an object of the invention is to provide an improved search coil assembly capable of being made in a manner greatly reducing the time required to produce such components, so that metal detectors of various sizes may be produced quickly, without the need for maintaining large inventories.
A further object of the invention is to provide a search coil assembly for metal detection devices, the search coil assembly being capable of being made by a method providing such economic benefits as to render purchase and use of such devices at the retail level practicable from an economic standpoint.